1. Field of the Invention
The present invention relates to a method of reducing flicker noises which are generated on a screen of an X-Y address type solid-state image pickup device by a fluorescent lamp for room illumination when photographing a room with the pickup device.
2. Description of the Related Art
Solid-state image pickup devices are recently used in an enormous quantity by incorporating them in various products such as digital still cameras, digital video cameras, and portable telephones. Solid-state image pickup devices are generally classified into CCD (charge coupled device) solid-state image pickup devices comprising a charge transfer type image sensor and X-Y address type solid-state image pickup devices in which an image sensor is constituted by a CMOS transistor (Complementary Metal Oxide Semiconductor). X-Y address type image pickup devices utilizing a CMOS image sensor are expected to replace CCD solid-state image pickup devices for the reason that they can be manufactured using a technique similar to a MOSFET manufacturing process; they can be driven by a single power supply with small power consumption; and various signal processing circuits can be loaded on the same chip.
A CMOS image sensor has a plurality of pixel regions which are connected to a plurality of vertical selection lines and horizontal selection lines and which are arranged in the form of a matrix. A photoelectric conversion device such as a photodiode is formed in each of the pixel regions. Light which has entered a light-receiving surface of each of the photoelectric conversion devices is subjected to photoelectric conversion, and an electric charge is accumulated in the device. The accumulated electric charge is converted by a source follower amplifier or the like provided in the pixel into a voltage which is then amplified and read out as image data for one pixel at predetermined timing.
Image data of a plurality of pixels connected to a predetermined horizontal selection line are output at a time in response to a line selection signal from a vertical scan shift register and are sequentially output to an external system based on a column selection signal from a horizontal scan shift register.
In an in-door photographic environment, a fluorescent lamp is often used as illumination. In Japan, the emission frequency of fluorescent lamps depends on districts. The emission frequency is 100 Hz (power supply frequency is 50 Hz) in some districts and 120 Hz (power supply frequency is 60 Hz) in some districts. FIGS. 6(a), 6(b), 7(a) and 7(b) show relationships between the emission frequencies of fluorescent lamps and a signal storage time of a conventional CMOS image sensor. Those figures indicate time along the abscissa axes and indicate quantities of light emitted by the fluorescent lamp along the ordinate axes. FIGS. 6(a) and 7(a) show a case wherein a fluorescent lamp having an emission frequency of 100 Hz (emission period (blinking period): 1/100 sec.) is used, and FIGS. 6(b) and 7(b) show a case wherein a fluorescent lamp having an emission frequency of 120 Hz (emission period (blinking period): 1/120 sec.) is used.
Referring to FIGS. 6(a) and 6(b), a description will now be made on signal storage performed by a photodiode at a pixel connected to a horizontal selection line in an x-th place from the beginning of a frame (hereinafter referred to as “x-th line”). A time at which signal storage is started on the x-th line is represented by 1xb; a time at which signal storage is terminated is represented by 1xe; and the signal storage time (integration time) is represented by ts.
For example, one frame period T is 1/30 (sec.) where frame period T is the sum of a vertical scanning period and a vertical blanking period required for the first horizontal selection line through the last horizontal selection line. Therefore, a frame frequency f is 30 Hz.
Since the signal storage time ts of a photodiode is a period required for reading image data after a reset pulse for resetting the photodiode is input, the signal storage time ts can be changed by changing the timing of the input of the reset pulse.
First, in the case of a fluorescent lamp having an emission period of 1/120 sec. as shown in FIG. 6(b), an integral multiple (four times) of the emission period of the fluorescent lamp corresponds to one frame period of a CMOS image sensor. Therefore, the signal storage starting time 1xb and signal storage ending time 1xe on the x-th line are the same timing relative to the emission periods of the fluorescent lamp in an n-th frame and an (n+1)-th frame. Thus, when an image is picked up under the fluorescent lamp having an emission frequency of 120 Hz, the brightness (the area of the hatched regions in the figure) of the image is constant between the frames.
In the case of a fluorescent lamp having an emission period of 1/100 sec. as shown in FIG. 6(a), an integral multiple of the emission period of the fluorescent lamp does not agree with one frame period of a CMOS image sensor, and there are 100/30≅3.3 periods per frame. Therefore, the signal storage starting time 1xb and signal storage ending time 1xe on the x-th line will not be the same timing relative to the emission periods of the fluorescent lamp in the n-th frame and (n+1)-th frame unless the signal storage time ts is adjusted to the emission period of the fluorescent lamp. Thus, when an image is picked up under the fluorescent lamp having an emission frequency of 100 Hz, the brightness of the image will be different between the frames.
A description will now be made on signal storage at pixels connected to different horizontal lines (an x-th line and a y-th line) in the same frame as shown in FIGS. 7(a) and 7(b). A time at which signal storage is started on the y-th line is represented by 1yb, and a time at which signal storage is terminated is represented by 1ye. A signal storage time ts is the same as that of the x-th line.
As apparent from the figures, the signal storage starting time 1xb and signal storage ending time 1xe for the x-th line and the signal storage starting time 1yb and signal storage ending time 1ye for the y-th line will not be the same timing relative to the emission periods of both of the fluorescent lamps having emission frequencies of 100 Hz and 120 Hz. Therefore, those lines appear with different brightness in the same frame under both of the fluorescent lamps having emission frequencies of 100 Hz and 120 Hz.
FIGS. 8(a) through 8(d) show a specific example of the phenomenon described above with reference to FIGS. 6 and 7. FIG. 8(a) illustrates an image in the n-th frame picked up under the fluorescent lamp having an emission frequency of 100 Hz, and FIG. 8(b) illustrates an image in the (n+1)-th frame. As shown in FIGS. 8(a) and 8(b), on the images picked up under the fluorescent lamp having an emission frequency of 100 Hz, a phenomenon is observed in which bright and dark horizontal fringes having a period of 3.3 appear in the screen and the fringes gradually move upward or downward. This phenomenon is flicker noises that appear in an image picked up under a fluorescent lamp having an emission frequency of 100 Hz.
FIG. 8(c) illustrates an image in the n-th frame picked up under the fluorescent lamp having an emission frequency of 120 Hz, and FIG. 8(d) illustrates an image in the (n+1)-th frame. As shown in FIGS. 8(c) and 8(d), bright and dark horizontal fringes in four periods which are stationary in the screen are observed in the images picked up under the fluorescent lamp having an emission frequency of 120 Hz. This phenomenon is flicker noises that appear on an image picked up under a fluorescent lamp having an emission frequency of 120 Hz.
Flicker noises occurs not only in a CMOS image sensor but also in a CCD image sensor. Since a CCD image sensor employs a unique method for reading image data, irregularity of luminance as shown in FIGS. 8(a) through 8(d) does not appear on the same screen. Therefore no flicker noise occurs under the fluorescent lamp having an emission frequency of 120 Hz. Further, only flicker noises that result in a difference in brightness between frames occur under the fluorescent lamp having an emission frequency of 100 Hz. A method for reducing such flicker noises will now be briefly described with reference to FIGS. 9(a) and 9(b). FIGS. 9(a) and 9(b) show relationships between emission frequencies of fluorescent lamps and a signal storage time ts of a conventional CCD image sensor. Those figures indicate time along the abscissa axes and indicate quantities of light emitted by the fluorescent lamps along the ordinate axes. FIG. 9(a) shows a case wherein a fluorescent lamp having an emission frequency of 100 Hz is used, and FIG. 9(b) shows a case wherein a fluorescent lamp having an emission frequency of 120 Hz is used.
According to a method of reducing flicker noises of a CCD image sensor, as shown in FIG. 9(a), the signal storage time ts (a signal storage stating time sb and a signal storage ending time se) in one frame is fixed to a value which will not result in flickers under the fluorescent lamp having an emission frequency of 100 Hz, e.g., three times the emission period 1/100 of the fluorescent lamp. When the signal storage time is preset at 3/100 sec., a fluorescent lamp can be used without flickers whether it has an emission frequency of 100 Hz or 120 Hz.
In the case of a CMOS image sensor, however, flickers occur at either of the emission frequencies 100 Hz and 120 Hz as described above, and a signal storage time ts resulting in no flicker at both of the emission frequencies 100 Hz and 120 Hz does not exist in a frame period of 1/30 sec. Therefore, the above-described method of reducing flickers in a CCD image sensor can not be used as it is in a CMOS image sensor.